1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal-combustion engine, and more particularly to an air-fuel ratio control device that is suitable as a device for controlling the air-fuel ratio of an internal-combustion engine having exhaust gas sensors at upstream and downstream of a catalyst positioned in an exhaust path.
2. Background Art
A related art internal-combustion engine that is disclosed, for instance, by Japanese Patent Laid-Open No. Hei7-197837 is equipped with two exhaust gas sensors that are mounted in an exhaust path of the internal-combustion engine. This internal-combustion engine is equipped with an air-fuel ratio sensor (sensor exhibiting a linear characteristic in relation to the air-fuel ratio), which is positioned upstream of a catalyst mounted in the exhaust path, and an oxygen sensor (sensor exhibiting a Z characteristic in relation to the air-fuel ratio), which is positioned downstream of the catalyst.
In the above related art internal-combustion engine, main feedback control is exercised in accordance with the output of the air-fuel ratio sensor, which is positioned upstream, and sub-feedback control is exercised in accordance with the output of the oxygen sensor, which is positioned downstream. In main feedback control, the fuel injection quantity is regulated so that the air-fuel ratio of an exhaust gas flowing into the catalyst coincides with a target air-fuel ratio. The catalyst has a catalyst window in which being capable of efficiently purifying the incoming exhaust gas. In other words, the catalyst exhibits a satisfactory purification characteristic when the air-fuel ratio of the incoming exhaust gas is within the catalyst window. The target air-fuel ratio is a predetermined value so as to be within the catalyst window. Therefore, if main feedback control is ideally exercised, the exhaust gas emitted from the internal-combustion engine is efficiently purified by the catalyst so that no unpurified component blows downstream of the catalyst.
However, a certain degree of error is superposed over the air-fuel ratio sensor output and various other elements involved in main feedback control. Therefore, even if main feedback control is exercised as described above, the air-fuel ratio of an exhaust gas flowing into the catalyst may, in reality, shift toward the rich side or lean side, thereby allowing an unpurified component to flow downstream of the catalyst. In the above-mentioned related art internal-combustion engine, sub-feedback control is exercised to correct a fuel injection quantity calculation method to avoid air-fuel ratio deviation, which causes an unpurified component flow, when an unpurified component flows downstream of the catalyst as described above. When such sub-feedback control is exercised, various influences of error can be corrected so as to accurately keep the actual air-fuel ratio close to the target air-fuel ratio.
However, the above-mentioned catalyst window may vary depending on the operation of the internal-combustion engine. In a situation where the main and sub-feedback control functions in such a manner that no unburned component flows downstream of the catalyst by a certain catalyst window, an unburned component may flow downstream of the catalyst if the catalyst window varies no matter whether the same feedback control is continuously exercised. Therefore, it is necessary in the related art internal-combustion engine described above to correct the contents of sub-feedback control for the purpose of complying with a renewed situation each time the operation thereof changes thereby the catalyst window varies. If the operation of the internal-combustion engine frequently changes, the corrections provided by sub-feedback control cannot comply with such a frequent change. As a result, the emission characteristic may deteriorate.